This invention relates generally to high-voltage electrical apparatus, and more particularly to an improved particle trapping system for use in gas insulated transmission lines, and to an improved method for conditioning gas-insulated transmission lines.
High-voltage gas-insulated transmission lines typically comprise an outer sheath at low or ground potential, an inner conductor at high potential with respect to the grounded outer sheath which is disposed within the outer sheath, and support insulators for insulatively supporting the inner conductor within the outer sheath. An insulating gas is generally utilized to electrically insulate the inner conductor from the outer sheath, with the result that the high dielectric strength of the insulating gas enables closer, more compact spacings between the inner conductor and the outer sheath. Sulfur hexafluoride has been utilized as the insulating gas for many reasons associated with its useful combination of vapor pressure, chemical stability, cost, electric strength, thermal conductivity, and non-toxicity, among other properties.
One problem which has arisen in the design of these high-voltage gas-insulated transmission lines is the effect of the mobile conducting or semi-conducting particle. These particles may be remaining in the line after assembly, or may be produced during operation, and can traverse between the outer sheath and the inner conductor to thereby cause sparking, corona, or can lead to flashovers and breakdown of the insulating gas or the insulating support. These contamination particles can lower the breakdown strength of the sulfur hexafluoride gas. In order to overcome these particles effects, it may be necessary to increase the size of the gas-insulated transmission line, or include within the transmission line means for eliminating or deactivating the conducting particles.
One means utilized in the prior art to deactivate these particles has been the use of low electric field regions as taught by Trump in U.S. Pat. No. 3,515,939. The low field regions are created by including within the transmission line conducting electrodes which are electrically connected to the outer sheath and which have portions thereof spaced therefrom, so that a low field region is formed between the electrode and the outer sheath. These electrodes are generally known as particle traps.
The principle of a particle trap is to create a region of very low or zero field in the compressed gas insulated transmission system into which particles can be moved by an electric field. The trap usually consists of a longitudinal metallic shield or cylinder mounted around the insulator inside and spaced-apart from the bottom of the outer sheath. Slots are provided in the bottom of the trap surface for particles to fall through and into the low field region where they are deactivated.
It has been found that, with such an elevated trap, most of the particle contaminates are captured by migrating through the opening between the trap and the enclosure when the applied 60 hertz voltage to the inner conductor is just above the voltage required for the particles to be lifted off the outer sheath surface. The activity and bounce height of the particles are then limited and the particles move and remain close to the outer sheath surface. This condition is most desirable for effective particle migration into the traps and because it reduces the chance of particles moving onto the insulator or moving to the high field conductor where they could cause breakdown. Once the particles are in the trap area, it is important that they be retained and not be permitted to escape again at a higher voltage, as this escape may result in breakdown and thus damage to the system requiring the transmission line section to be opened and repaired.